The present invention relates to steam turbine blades and, more particularly, to side entry turbine blade roots.
Side entry turbine blade roots are typically formed with Christmas tree-shaped roots which fit into correspondingly shaped grooves in a rotor disk. The roots generally have three lugs on each side of a root centerline. Each lug has an inclined bearing surface which bears against a groove so that each blade root reacts against six bearing surfaces. When each blade is considered as a separate and distinct entity, the blade root surfaces and the groove bearing surfaces can be formed for satisfactory mating relationships and thus provide the desirable and required support for the blade.
It has become common practice to join individual blades into groups of blades by attachment to common platform and/or shroud portions. Such multiple blade units have higher rigidity and lower vibration susceptibility than single blades. In one form, a blade group may be constructed by attaching radially outer ends of several blades to a shroud after the blade roots are inserted into their respective rotor grooves. A disadvantage of coupling individual blades into groups is that circumferential displacement of the blades by the coupling tends to misalign the roots within the grooves. If the blade root centerline is not aligned with the groove centerline, the bearing surfaces may not seat properly causing stresses on the root structure to be unevenly distributed. In some instances, it has been found that some of the lugs lose contact with the groove surfaces so that only a portion of the lugs carry the blade stress. Such uneven loading can result in cracking and eventual failure of the root with potential blade separation during turbine operation.
In another form, blade groups may be constructed as integral units having a common shroud and a common platform. Such a blade group is illustrated in U.S. Pat. No. 4,130,379 to Partington and assigned to the assignee of the present invention. In this form, the blade roots may also be circumferentially displaced such that the blade root centerlines do not coincide with radius lines of the turbine rotor in which the blades are installed. This will result in similar shifting of the load bearing surfaces of the lugs on the blade root causing one or more lugs to carry more than their proportionate share of the blade loading and be subject to stress cracks and potential failure.